power supply

Hi all,

I have just made a project that displays the temperature from two sensors on a LCD display . My doubt is ,since the circuit is designed to work on 12 v batteries ( 2 x9v batteries in series limited by a LM7812 ic) , Can it be connected to a car battery without any modification or will the high amperage car battery blow up the components.
My friend tells me that the system will draw only the required amps from the car battery is this true ?

thanks

Yes, quite true, circuits only take what they need - the supply current is only relevent if it can't supply enough current.

For a simple example!.

Consider your cold water tank at home - it probably holds 20-40 gallons of water - so how come you can fill a half pint glass and not get wet feet? (VERY wet feet!). So the tank is the battery, and the glass your circuit.

Now if the tank is almost empty (only quarter of a pint), and you try and fill the half pint glass, it will only be half full - this is like the tank (battery) can't supply enough water (current).

Water analogies work quite well for electronics 8)

The voltage of a car battery can be somewhere between 11.5 and 14 or so volts, disconnected or with the engine off. During cranking it can drop below that range and while charging it might be above that range. Auto systems are also loaded with noise and voltage spikes so instantaneous voltage can be much higher.

If you leave the 7812 in the circuit it will only function to reduce the high range of the voltage. As I understand it there is a voltage drop or loss that normally requires the input voltage to the 7812 to be several volts above the desired regulated output voltage. If your circuit really does need 12 volts you might just get 12 volts out of the 7812 when the system voltage is at 14 volts or above, otherwise it will fall short. I am not sure if this will affect your circuit.

To what degree the 7812 will suppress voltage spikes is unknown to me. You might add a 15 volt or higher zener across the input and give it a try. I don't know that an undervoltage condition would do any harm though the data (reading) might appear good and yet be unreliable. Some electronic devices will only function reliably with the nice, smooth, noise free power of a battery so best to experiment.

If your device does need 12 volts you can get closer to it with a low dropout regulator if the current requirements are less than the particular regulator. If your device will function reliably at let's say 10 volts you might use an LM317 ( or better yet a low dropout regulator) to supply a steadier voltage more of the time. A switched mode regulator would ultimately the the best as long as the output were properly filtered.

The water analogy is the best. In case it didn't sink in just consider that small, low wattage, mains (120 or 220 vac) light bulbs are in your light fixtures connected to the same source (mains) as other high current loads. Until the fuse or circuit breaker opens there can be 10,000 or much more available amps (short circuit) yet things function just fine.

sorry

hi all,


I am using a 7805 instead of a 7812 , it was a typing error . I guess this should solve the problem of the input voltage being above the regulated voltage. (desired voltage)

Thanks for the help

Re: sorry

dreamproject said:

hi all,


I am using a 7805 instead of a 7812 , it was a typing error . I guess this should solve the problem of the input voltage being above the regulated voltage. (desired voltage)

Thanks for the help

Click to expand...

So you wont have a regulation drop out problem but it might not be your only concern. As I understand it, if this is installed in a running car environment, you can expect to see some pretty nasty voltage transients many 10's (possibly 100's) of volts on your input momentarily. I would put at least a 20V clamp (zener or otherwise) on the input.

If you are talking about the car battery just sitting on the ground outside of the running environment, then you're probably ok with what you have.

As mentioned above, you will need noise suppression in a vehicle.

If it is used in a vehicle, a MOV across the supply line helps alot also. In your case, something like a S14K14 by Epcos would do the job. they are about 50 cents US.

Everything we install in a vehicle at work gets atleast one in the power supply section, and we have had good success with them.